Bright zinc electroplating



Patented June 10, 1952 UNITED STATES TENT OFFICE signor fto I. an Pontde Nemours & Company, Wilmington, DeL, a corporation of Delaware NDrawing. Application January 7,1950, Serial No. 137,483

4 Claims. 1

This invention relates to the electrodeposition of zinc, and moreparticularly to a method for electroplating bright zinc from a 'zinccyanide solution.

Zinc electroplated onto steel or other base metal from a zinc cyanideelectrolyte without the use of addition agents generally has anundesirable dark grey appearance. Bright Zinc deposits can be obtainedby adding to the electroplating bath various know'n'addition agents butgenerally in such cases, the bright deposit is covered by a thinbrownish film'which has to be removed by a subsequent operationconsisting of dipping the electroplated article into a dilute solutionof an oxidizing agent. Acetone and certain other ketones have beenproposed as addition agents for zinc plating. These have not proved toexcel other known brighteners and also require the bright dipping aftertreatment.

An object of the present invention is to provide an improved method forelectroplating bright zinc onto steel and other base metals. A furtherobject is to electrodeposit bright zinc which is free from the usualbrownish film or other discoloration and which does not have to betreated after electroplating in order to obtain a filmfree, bright,smooth surface. Still other objects will be apparent from the followingdescription of the invention.

The above objects may be attained in accordance with the presentinvention by electroplating zinc from a zinc cyanide solution whichcontains a small amount of an alkali metal stannate together with asmall amount of one ormore of certain ketones. The ketones suitable forpracticing the invention are methyl gamma-hydroxypropyl ketone, methyln-propyl ketone, methyl ethyl ketone, acetone, diacetone alcohol, acetylacetone, mesityl oxide, cyclopropyl methyl ketone and cyclohexanone. Ofthese, the preferred ketone is methyl gamma-hydroxypropyl ketone. Theother ketones mentioned above are efiective brighteners only when usedin conjunction with an alkali metal stannate. In the absence of thestannate they have only a comparatively slight brightening effect onthe-electroplated zinc and form the usual film which must be removed bybright dipping. On the other hand, the methyl gamma-hydroxypropyl ketonegives excellent results when used alone, producing a clear, brightelectrodeposit which is filmfree and needs no after treatment. The bestresults have been obtained by using methyl gamma-hydroxypropyl ketone inconjunction with an alkali metal stannate, which-produceselectrodepos'its of great brilliancy.

In place of the above ketones, I may use their cyanhydrins, as'thecyanhydrins readilyhydrolize in the aqueous plating bath to set free the"ketones therein. Thus, one method of adding a ketone to theelectroplating bath is to add its cyanhydrin, which immediately-formsthe-ketone in situ. 7

The invention is illustrated by the following examples:

Example '1 Aqueous zinc cya'nidelectroplating solutions were preparedhaving the following "compositions:

Zn(CN) 2 ag./l. NaCN 5060'g./1. NaOI-I g./l. NazS 0 5'g;/l. Na2SnO3.3HzO-1;5g./1.

In a series of electroplating operations utilizing electrolytes of theabove composition, with the further addition in each case of a ketone asindicated in the table below, sheet steel electrodes were electroplatedwith zinc atcurrent densities of 2 to amps/sq. ft. and at a bathtemperature of 25 to 30 C.

Each ketone was tested at several concentrations, an indicated inthe'above table under Ketone concentration. Within the ketoneconcentration ranges shown, excellent bright 'zinc deposits of highbrilliancy were obtained "withinl-the current density rangesasindicatedin the table. All the deposits were -free'from film andrequired no after treatment of any kind.

Example 2 Zinc was electrodeposited from an electrolyte like that ofExample '1," except that it contained no sodium stannate. Withthe'f'platin'g conditions of Example 11 this stannate" free bath iasoperated in separate trials with the addition of difierent ketones, asshown below:

Ketone Kewne Concentration Acetone SccJl. Acetyl Acetone 10 cc./l.

In each case the resulting electrodeposit was coated with a brown film.On dipping in a dilute nitric acid solution, the film was removedproducing a bright plate, but less bright than that obtained with theseketones in Example 1.

Example 3 Zinc was electroplated onto steel crank hole covers from theaqueous electrolyte given below using gamma-hydroxypropyl methyl ketonewithout sodium stannate addition:

Cathode current densities varied from 10 to 30 amps/sq. ft. The solutiontemperature was 25 0. Bright zinc deposits which required no brightdipping were secured at current densities of 18 to 30 amps/sq. ft.

Example 4 The following ketones were tested by the procedure of Example1 and produced the results tabulated below:

Ketone Concentration Results Ketone Methyl Isobutyl Kc 5, l and 15 g.il.Dark colored deposit.

tone

Hydroquinone 4, 8 and 12 g./l Dull deposit. Di-tetrahydrofuryl gjl Brownfilm at 30 amps./ Pentauone. sq. ft. and dull plate at 1-20 amps/sq. it.Methyl vinyl ketone.- 4-8 cc./l Bright plate at first,

but ketone polymerizes making solution inoperable.

In practicing the invention to obtain the desired bright zinc depositsthe alkali metal stannate and ketone are added to the electroplatingbath from time to time as required, to maintain therein an alkali metalstannate concentration equivalent to 1 to 2.5 grams per liter of sodiumstannate and a dissolved ketone concentration of approximately 3 to 26grams per liter. When the methyl gamma-hydroxypropyl ketone is used,either with or without stannate addition, its concentration in theplating bath preferably is maintained at 5 to 15 grams per liter. Inconcentrations below 1 gram per liter the effect of the stannate isrelatively small; and in order to obtain a film-free bright deposit, thestannate concentration should equal at least 1 gram per liter. Atconcentrations above 2.5 grams per liter, the stannate causes theformation of a dull plate, destroying the brightening eifect of theketone. At concentrations within the range of 1 to 2.5 grams per liter,the stannate enhances the brightening effect of the ketone and preventsfilm formation.

A concentration of at least about 3 grams per liter of the ketone isrequired to obtain a commercially acceptable bright plate, althoughsmaller amounts have more or less brightening effect. While amountslarger than 26 grams per liter may be used with good results (it theketone is sumciently soluble), generally the results are no better thanat 3 to 26 grams per liter. As the ketone must go into solution tofunction as a brightener, I prefer a ketone concentration not exceedingthe solubility limit. The solubilities of the ketones used herein varyconsiderably. Thus, acetone is soluble throughout the range of 3 to 26grams per liter, while other ketones such as methyl gamma-hydroxypropylketone is not- However, adding the ketone in amount in excess of itssolubility in the plating bath does no harm within the limits of 3 to 26grams per liter.

7 While generally it is preferred to use sodium stannate as additionagent in practicing the invention, potassium stannate and other alkalimetal stannates may be used with equally good results at the same molarconcentration.

In practicing the present invention it is preferable to utilize aselectrolyte a zinc alkali metal cyanide solution such as thatillustrated by the above examples which contains free cyanide and addedalkali metal hydroxide. Also the best results are secured if the bath ispurified as required by the addition of sodium sulphide or other alkalimetal sulphide, zinc dust, or the like, which serves to precipitate outany heavy metal impurities which may be introduced into the bath fromthe zinc anodes or from the zinc cyanide. The invention, however, is notrestricted to these preferred solutions but may be carried out with anysolution of a zinc alkali metal cyanide.

The electroplating conditions such as bath temperature, current density,filtration of electrolyte, choice and location of anodes, and the likemay be those conventionally employed in cyanide zinc electroplatingoperations. The herein described invention permits the production ofexcellent bright zinc electrodeposits over a wide range of currentdensities, e. g., from 2 to amps/sq. ft. and higher.

I claim:

1. The process which comprises electroplating zinc from an aqueous zinccyanide solution which contains about 60 grams per liter of zinc cyanideand an alkali metal stannate in amount equivalent to 1 to 2.5 grams perliter of sodium stannate and to which has been added about 3 to 26 gramsper liter of a ketone selected from the group consisting of acetone,methyl ethyl ketone, methyl n-propyl ketone. gamma-hydroxypropyl methylketone, acetyl acetone, diacetone alcohol, mesityl oxide and cyclopropylmethyl ketone.

2. The process which comprises electroplating zinc from an aqueous zinccyanide solution which contains about 60 grams per liter of zinc cyanideand 1 to 2.5 grams per liter of sodium stannate and to which has beenadded about 3 to 26 grams per liter of a ketone selected from the groupconsisting of acetone, methyl ethyl ketone, methyl n-propyl ketone,gamma-hydroxypropyl methyl ketone, acetyl acetone, diacetone alcohol,mesityl oxide and cyclopropyl methyl ketone.

3. The process which comprises electroplating zinc from an aqueous zinccyanide solution which contains about 60 grams per liter of zinc cyanideand 1 to 2.5 grams per liter of sodium stannate and to which has beenadded about 3 to 26 grams per liter of acetone.

4. The process which comprises electroplating zinc from an aqueous zinccyanide solution which contains about 60 grams per liter of zinc cyanideand 1 to 2.5 grams per liter of sodium 5 stannate and to which has beenadded about 5 Number to 15 grams per liter of gamma-hydroxypropyl1,904,732 methyl ketone. 1,905,105 CHRISTIAN J. WERNLUND. 2,109,887

REFERENCES CITED 0 The following references are of record in the ggggfile of thls patent. 548:009

UNITED STATES PATENTS 10 Number Name Date 736,565 Szirmay Aug. 18, 1903Name v Date Haneisen et a1. Apr. 18, 1933 Kersten Apr. 25, 1933Mattacotti Mar. 1, 1938 FOREIGN PATENTS Country Date Great Britain Oct.'7, 1937 Great Britain Sept. 21, 1942

1. THE PROCESS WHICH COMPRISES ELECTROPLATING ZINC FROM AN AQUEOUS ZINCCYANIDE SOLUTION WHICH COMTAINS ABOUT 60 GRAMS PER LITER OF ZINC CYANIDEAND AN ALKALI METAL STANNATE IN AMOUNT EQUIVALENT TO 1 TO 2.5 GRAMS PERLITER OF SODIUM STANNATE AND TO WHICH HAS BEEN ADDED ABOUT 3 TO 26 GRAMSPER LITER OF A KETONE SELECTED FROM THE GROUP CONSISTING OF ACETONE,METHYL ETHYL KETONE, METHYL N-PROPYL KETONE, GAMMA-HYDROXYPROPYL METHYLKETONE, ACETYL ACETONE, DIACETONE ALCOHOL, MESITYL OXIDE AND CYCLOPROPYLMETHYL KETONE.